1. Field of the Invention
The present invention relates generally to automatic focusing apparatuses, and more specifically, to an automatic focusing apparatus for automatically matching the focus to an object in response to a video signal obtained from an image sensor, in an image sensing apparatus such as a video camera having an automatic focusing function.
2. Description of the Background Art
Conventionally, in an automatic focusing apparatus for use in an image sensing apparatus such as a video camera, an approach utilizing a video signal obtained from an image sensor itself for evaluation of the state of focus control has been developed. Such an approach is essentially free from parallax and possesses excellent characteristics that, for example, the focus can exactly be matched even if the depth of field is small and an object is located in the distance. Furthermore, according to this approach, a special sensor for automatic focusing does not have to be separately provided and therefore the apparatus is of a very simple mechanism.
As an example of such a focus control method utilizing a video signal, a so-called hill-climbing servo system has been conventionally known. An automatic focusing apparatus utilizing the hill-climbing servo system is disclosed, for example, in U.S. Pat. Nos. 4,922,346 and 5,003,339. Briefly stated, a high frequency component of a video signal from an image sensor is detected for every field as a focus evaluating value, the detected focus evaluating value is always compared with a focus evaluating value detected one field before, and the position of a focusing lens continues to be slightly vibrated so that the focus evaluating value always takes the maximum value.
FIG. 7 is a block diagram schematically showing a conventional automatic focusing apparatus using such a hill-climbing servo system, and FIG. 8 is a block diagram showing in detail a focus evaluating value generation circuit in FIG. 7.
In FIG. 7, a video camera includes a focus ring 2 for advancing/withdrawing a focus lens 1, a focus motor 3 for driving focus ring 2, and an image-sensing circuit 4 including an image sensor such as a CCD (not shown). Advancing/withdrawing of focus lens 1 may be performed using a piezoelectric element instead of the motor, and the image sensor itself such as a CCD (not shown) may be advanced/withdrawn rather than focus lens 1.
An image formed by focus lens 1 on the plane of the image sensor is converted into an image sensed video signal by image sensing circuit 4, to be recorded on a magnetic recording medium which is not shown or externally output, and to be input to focus evaluating value generating circuit 7 as well.
Referring to FIG. 8 showing in detail focus evaluating value generating circuit 5, a luminance signal component in the video signal output from image sensing circuit 4 is applied to a synchronizing separation circuit 5a and a high-pass filter (HPF) 5c. A high frequency component of the video signal extracted by HPF 5c is amplitude-detected by a detection circuit 5d, and the detection output is applied to an A/D conversion circuit 5e. A/D conversion circuit 5e converts the applied detection output into a digital value for supply to a gate circuit 5f.
Meanwhile, synchronizing separation circuit 5a separates a vertical synchronizing signal and a horizontal synchronizing signal from the input luminance signal and applies the separated signals to a gate control circuit 5b. Gate control circuit 5b sets a rectangular sampling area in the center of the picture plane based on the input vertical and horizontal synchronizing signals and a fixed output from an oscillator (not shown).
Gate control circuit 5b applies to gate circuit 5f a signal for opening/closing the gate for every field so as to permit the passage of the output of A/D conversion circuit 5e only within this sampling area. It is noted that gate circuit 5f may be provided in any place in preceding stages up to an integrating circuit 5g which will be described later.
By the provision of gate circuit 5f, only the A/D conversion value of a high frequency component corresponding to the range of sampling area is applied to integrating circuit 5g. Integrating circuit 5g integrates the applied A/D conversion values for every field in a digital manner and supplies the resultant integration value as the present focus evaluating value.
Now, an operation immediately after automatic focus control is initiated will be described with reference to FIG. 7. Immediately after initiation of an automatic focusing operation, a focus evaluating value for the first 1 field output from focus evaluating value generating circuit 5 is applied to a maximum value memory 6 and an initial value memory 7 and held therein. Thereafter, a focus motor control circuit 10 generates a signal MS instructing on/off of focus motor 3 and a signal DS instructing the direction of rotation of focus motor 3 and applies the generated signals to a focus motor driving circuit 31, thereby rotating focus motor 3 in a prescribed direction. Then, a comparator 9 compares the initial focus evaluating value held in initial value memory 7 with the present focus evaluating value output from focus evaluating value generating circuit 5 to generate a comparison signal, and focus motor control circuit 10 responds thereto and performs an initialization for the direction of rotation of focus motor 3.
More specifically, focus motor control circuit 10 rotates focus motor 3 in the above-stated prescribed direction until comparator 9 generates a comparison output indicating "large" or "small". Then, if the comparison output is supplied from comparator 9 indicating that the present focus evaluating value is larger than the initial focus evaluating value held in initial memory 7 by more than a prescribed first threshold value, focus motor control circuit 10 maintains the above-stated prescribed rotation direction. Meanwhile, if the comparison output is supplied from comparator 9 indicating that the present focus evaluating value is smaller than the initial focus evaluating value by more than the first threshold value, focus motor control circuit 10 reverses the rotation direction of focus motor 3.
Thus, the initialization for the rotation direction of focus motor 3 is completed, and focus motor control circuit 10 then monitors the output of comparator 8. It is noted that during the period in which the difference between the initial focus evaluating value and the present focus evaluating value does not exceed the prescribed first threshold value, comparator 9 does not generate a comparison output indicating large or small, so that an erroneous operation due to noise in the focus evaluating values can be prevented.
Meanwhile, comparator 8 compares the largest prior focus evaluating value held in maximum value memory 6 with the present focus evaluating value output from focus evaluating value generating circuit 5, and outputs two kinds of comparison signals (S.sub.1, S.sub.2) indicating that the present focus evaluating value is larger than the focus evaluating value held in maximum value memory 6 (first mode) and that it is reduced by more than a prescribed second threshold value (second mode), respectively. If the present focus evaluating value is larger than the content of maximum value memory 6, the content of maximum value memory 6 is updated in response to the output (S.sub.1) of comparator 8; thus, the current maximum value for the focus evaluating value is always held in maximum value memory 6.
A focus motor position signal is generated from a motor position detecting circuit 30 in response to the position of focus motor 3 driving focus ring 2 which supports focus lens 1, and the focus motor position signal is applied to a focus motor position memory 13. Focus motor position memory 13 is updated so as to always hold the focus motor position signal obtained when the focus evaluating value takes the maximum value, in response to the output of comparator 8.
Focus motor control circuit 10 monitors the output of comparator 8 while rotating focus motor 3 in the direction initialized based on the output of comparator 9 as described above. If the comparison output (S.sub.2) of the second mode is obtained from comparator 8 indicating that the present focus evaluating value is reduced as compared with the maximum focus evaluating value by more than the above-stated second threshold value, focus motor control circuit 10 reverses the rotation direction of focus motor 3. The direction of rotating focus motor 3 is reversed for the first time only in response to the reduction by more than the prescribed second threshold value in order to prevent an erroneous operation due to noise in the focus evaluating values.
Thus, reversing the rotation direction of focus motor 3 changes the direction of the movement of focus lens 1 from the direction approaching the image sensor to the direction away therefrom or vice versa.
After focus motor 3 is reversed, the content of focus motor position memory 13 corresponding to the maximum focus evaluating value and the present focus motor position signal generated from focus motor position detecting circuit 30 are compared with each other at a comparator 14. If they match with each other, in other words if focus ring 2 returns to the position where the focus evaluating value takes the maximum value, focus motor control circuit 10 stops the rotation of focus motor 3. At the same time, focus motor control circuit 10 outputs a lens stop signal LS. Thus, a series of steps in the automatic focusing operation is completed.
A memory 11 and a comparator 12 are circuits for resuming an automatic focusing operation by focus motor control circuit 10 when the focus evaluating value is changed by more than a prescribed third threshold value at the time of the focus lens being stopped. More specifically, the focus evaluating value at the time when the focusing operation by focus motor control circuit 10 is completed and the lens stop signal LS is generated is held in memory 11. Then, the content of memory 11 and the present focus evaluating value output from focus evaluating value generating circuit 5 are compared with each other by comparator 12, and if the difference between them is larger than the prescribed third threshold value, an object changing signal is applied to focus motor control circuit 10, assuming that the object has been changed. Consequently, an automatic focusing operation by focus motor control circuit 10 is resumed, and a highly accurate automatic focusing operation following the change in the wide range of the object is realized.
The above-described automatic focusing apparatus is however still encountered with the following disadvantage.
When an automatic focusing operation is initiated, as described above, focus motor 3 is first rotated in a prescribed direction, in order to initialize the direction of rotation of focus motor 3. In this case, the abovedescribed first threshold value is preferably set large in order to prevent an erroneous operation due to the noises in the focus evaluating values and to secure the reliability of the operation. However, if the first threshold value is set large, the position of the focus lens is moved greatly away from the in-focus position when the initial rotation direction is opposite from the true direction of focusing.
A focus evaluating value input from focus evaluating value generating circuit 5 to comparator 9 has a delay from the time point of actual light incidence to the image sensor due to accumulation of optical charges in the image sensor and an integration for 1 field period in focus evaluating value generating circuit 5. Such time delay is usually 1.5 field period in total corresponding to addition of 1 field period necessary for integration of the output of the image sensor to 0.5 field period which is average for the periods for accumulating the optical charges caused by incident light in the image sensor at a certain time point. Therefore, if the focus evaluating value is monitored while rotating focus motor 3 as described above, a reversed movement of the focus lens from the in-focus position is further increased due to the above-described time delay from the time point of actual light incidence to the generation of the focus evaluating value.
These problems are not only encountered in initializing the in-focus direction, but also in detecting the in-focus position. More specifically, when the in-focus position is detected, the above-described second threshold value is also preferably set large in order to prevent an erroneous operation due to the noises in the focus evaluating values. If the second threshold is set large, however, the focus lens will go far beyond the in-focus position. The above-stated delay of the focus evaluating value accelerates this going beyond of the lens.
In order to prevent such reversed movement or going beyond of the position of a focus lens, an approach is suggested which includes driving a focus motor so as to make focus lens repeat a reciprocating rotational operation, detecting the phase of a modulated component for a focus evaluating value generated by the reciprocating operation of the focus lens with respect to the period of the above-stated reciprocating operation as a reference, and determining an in-focus direction based on the detected phase, for example, as described in Japanese Patent Laying-Open No. 58-215873 (H04N 5/26). Such an approach however requires the the focus lens be reciprocating many times in order to improve the reliability of the determination of the in-focus direction, and suffers from a disadvantage that it takes time for performing an automatic focusing operation even if the operation is initiated from a position relatively close to the in-focus position.
If light illuminating an object flickers like a fluorescent lamp, the focus evaluating value fluctuates in relation to the field frequency of a video signal. Accordingly, a method including a reciprocating operation of a focus lens always at a constant frequency as described above has a disadvantage that determination of the in-focus state can not be reliably secured. For example, assuming that a fluorescent lamp flickers at a flickering frequency of 100 Hz at a power supply frequency of 50 Hz, since the field frequency for a video signal of NTSC system is 60 Hz, the magnitude of the focus evaluating value, in other words a luminance on a reproduced picture plane periodically vibrates at a frequency of 20 Hz (in other words at a 3 field cycle) due to the deviation between the two frequencies. Accordingly, the periodic change of the focus evaluating value due to the reciprocating operation of the focus lens described above is affected by the oscillates of 20 Hz due to the flickering of the illuminating light, thus making it difficult to accurately determine the direction of focusing.
More specifically stated, unless the period of the reciprocating operation of the focus lens is integral multiples of or 1/integral multiples of 20 Hz, an actually produced focus evaluating value is sometimes increased when the focus lens is moved in the direction in which the focus evaluating value is supposed to be reduced, while an actually produced focus evaluating value is reduced when the lens is moved in the direction in which the focus evaluating value is supposed to be increased, and therefore the reliability of determination of the in-focus state can not be secured.